System and method for estimating a frequency of slider airbearing resonance

ABSTRACT

A method and apparatus for estimating the value of a slider airbearing resonance frequency involves obtaining a readback signal from a data storage medium over a plurality of complete airbearing periods and estimating the value of an airbearing resonance frequency using the readback signal. In one embodiment, a discrete signal segment comprising a plurality of frequency transform components is produced using the readback signal information, and the value of the airbearing resonance frequency is estimated using spectral leakage in the discrete signal segment. A ratio of the magnitudes of a first DFT component to a second DFT component is computed at each of a plurality of sampling rates. Each of these sampling rates is defined by a number of samples per average airbearing cycle multiplied by a frequency falling within a range of expected airbearing frequencies associated with a given implementation. The second DFT component is related to the slider airbearing resonance frequency, and the first DFT component is a DFT component adjacent to or non-adjacent to the second DFT component. The airbearing resonance frequency value is estimated using a minimum of the ratios, which may also constitute DFT component power ratios. A number of different frequency transform techniques may be employed, including Discrete Fourier Transform, Fast Fourier Transform, and Short-Time DFT techniques. One of several frequency transform approaches may be implemented depending on whether the detected airbearing signal is stationary or non-stationary. The airbearing resonance frequency methodology may be implemented in-situ a data storage system.

FIELD OF THE INVENTION

[0001] The present invention relates generally to data storage systemsand, more particularly, to a system and method for estimating afrequency of slider airbearing resonance.

BACKGROUND OF THE INVENTION

[0002] Within the data storage system manufacturing industry, muchattention is presently being focused on reducing head-to-disk clearanceas part of an effort to increase the storage capacity of data storagedisks. It is generally desirable to reduce the head-to-disk clearance inorder to increase the readback signal sensitivity of the transducer totypically weaker magnetic transitions associated with higher densitydisks. When decreasing the head-to-disk clearance, however, theprobability of detrimental contact between the sensitive transducer andan obstruction on the disk surface significantly increases. Ashead-to-disk clearance continues to decrease, it becomes increasinglyimportant to assess the general health of each read/write head,including flying characteristics, during the operating life of a datastorage system.

[0003] A prevalent surface irregularity that afflicts an appreciablepercentage of conventional data storage disks is generally referred toas an asperity. Asperities are isolated submicron-sized particles,typically comprising silicon carbide material, that are embedded in thedisk substrate. No single mechanism has yet been identified as thesource of such asperities, and it is believed that asperity defectsarise from numerous sources. Such asperities are often large enough tointerfere with the flight path of a typical slider/transducer assemblyby physically impacting with the slider/transducer assembly at a veryhigh velocity.

[0004] Further, asperities arising from the surface of a data storagedisk are generally distributed in a highly random manner, and change inshape and size in response to changes in disk and ambient temperatures.A collision between a slider/transducer assembly and an asperity oftenrenders the location of the asperity unusable for purposes of readingand writing information. Moreover, repeated contact between theslider/transducer assembly and asperity may cause damage of varyingseverity to the slider/transducer assembly.

[0005] Magneto-resistive (MR) transducers, for example, are particularlysusceptible to interference from contact with asperities. It iswell-known that MR transducers are very sensitive to variations intemperature, and are frequently used as temperature sensors in otherapplications. A collision between an MR transducer element and anasperity results in the production of heat, and a corresponding rise intransducer element temperature. Such transient temperature deviationsare typically associated with an inability of the MR transducer elementto read previously written data at the affected disk surface location,thereby rendering the stored information unrecoverable. An increase inthe frequency of head-to-disk contact events may be indicative of a headthat is flying lower than its intended average flyheight.

[0006] In the continuing effort to minimize head-to-disk clearance,manufacturers of disk drive systems recognize the importance ofdetecting changes in the flying characteristics of each individualread/write head during manufacturing and, importantly, during use of thedisk drive system in the field. There exists a need in the data storagesystem manufacturing community for an apparatus and method for detectingchanges in head flyheight. There exists yet a further need to providesuch an apparatus and method which is suitable for incorporation intoexisting data storage systems, as well as into new system designs, andone that operates fully autonomously in-situ a data storage system. Thepresent invention is directed to these and other needs.

SUMMARY OF THE INVENTION

[0007] The present invention is directed to a method and apparatus forestimating the value of a resonance frequency of an airbearingassociated with a slider flying in proximity to a data storage medium.Estimating an airbearing resonance frequency according to the presentinvention involves obtaining a readback signal from a data storagemedium over a plurality of complete airbearing periods and estimatingthe value of an airbearing resonance frequency using the readback signalinformation.

[0008] In accordance with one embodiment, a discrete signal segmentcomprising a plurality of frequency transform components is producedusing the readback signal information, and the value of the airbearingresonance frequency is estimated using spectral leakage in the discretesignal segment. A number of different frequency transform techniques maybe employed, including Discrete Fourier Transform (DFT), Fast FourierTransform (FFT), and Short-Time DFT (STFT) techniques, for example. Oneof several frequency transform approaches may be implemented dependingon whether the detected airbearing signal is stationary ornon-stationary over time.

[0009] In accordance with another embodiment, a ratio of the magnitudeof a first DFT component of the discrete signal segment to the magnitudeof a second DFT component is computed at each of a plurality of samplingrates. Each of these sampling rates is defined by a number of samplesper average airbearing cycle multiplied by a frequency falling within arange of expected airbearing frequencies associated with a given designor implementation. The second DFT component is related to the resonancefrequency of the slider airbearing, and the first DFT component is a DFTcomponent preferably adjacent to the second DFT component. The first DFTcomponent may alternatively be a DFT component non-adjacent to thesecond DFT component. The resonance frequency value of the sliderairbearing may be estimated using a minimum of the ratios. According toanother embodiment, the minimum of a number of first and second DFTcomponent power ratios may be used to estimate the value of theairbearing resonance frequency.

[0010] The discrete signal segment is preferably produced in response todetecting contact between the slider and a feature protruding from asurface of the data storage medium. The readback signal may comprise amagnetic signal component, a thermal signal component, or magnetic andthermal signal components. Other signal forms, such as optical signals,may also be processed by a method and apparatus according to the presentinvention. Goertzel's algorithm may be employed to compute themagnitudes of the first and second DFT components of the discrete signalsegment.

[0011] A method and apparatus for estimating a resonance frequency of anairbearing according to the principles of the present invention may beimplemented in a data storage system and, preferably, implementedin-situ a data storage system without resort to circuitry external tothe data storage system.

[0012] The above summary of the present invention is not intended todescribe each embodiment or every implementation of the presentinvention. Advantages and attainments, together with a more completeunderstanding of the invention, will become apparent and appreciated byreferring to the following detailed description and claims taken inconjunction with the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

[0013]FIG. 1 is a top perspective view of a disk drive system with itsupper housing cover removed;

[0014]FIG. 2 is a side plan view of a disk drive system comprising aplurality of data storage disks;

[0015]FIG. 3 is a partial sectional side view of an air bearing surfaceof a slider supported on an air bearing above a surface of a datastorage disk, the surface of the disk including a defect or obstruction;

[0016]FIG. 4 is a graphical illustration of a sinusoidal signal having afrequency, f₀ and sampled at a sampling rate, f_(s), where the samplingrate, f_(s), is eight times the sinusoidal signal frequency, f₀;

[0017]FIG. 5 depicts the magnitude of the Discrete Fourier Transform ofthe sinusoidal signal shown in FIG. 4;

[0018]FIG. 6 is a graphical illustration of a sinusoidal signal having afrequency, f₁, which is different from the frequency, f₀, of the signalshown in FIG. 4, the signal being sampled at the same sampling rate,f_(s), as that of FIG. 4;

[0019]FIG. 7 illustrates the magnitude of the corresponding DiscreteFourier Transform of the signal shown in FIG. 6, and further illustratesthe presence of spectral leakage;

[0020]FIG. 8 is a graphical illustration of a sinusoidal signal having afrequency, f₂, which is different from frequencies f₀ and f₁ of thesignals shown in FIGS. 4 and 6 respectively, the signal being sampled atthe same sampling rate, f_(s), as that of FIGS. 4 and 6;

[0021]FIG. 9 illustrates the magnitude of the corresponding DiscreteFourier Transform of the signal shown in FIG. 8, and further illustratesthe presence of spectral leakage;

[0022]FIGS. 10 and 11 respectively illustrate the magnitudes of two DFTcomponents adjacent a main DFT component for five complete periods of asinusoidal signal as a function of the ratio of sampling rate, f_(s), toconstant sinusoidal frequency, f₀;

[0023]FIG. 12 is a graphical illustration of a filtered and sampledreadback signal, w(m), depicting resonance of the slider airbearingassociated with a head-to-disk contact event occurring in anexperimental bump disk drive apparatus; and

[0024]FIG. 13 is a block diagram of a system for estimating a sliderairbearing frequency, f_(air), in-situ a data storage system inaccordance with an embodiment of the present invention.

[0025] While the invention is amenable to various modifications andalternative forms, specifics thereof have been shown by way of examplein the drawings and will be described in detail hereinbelow. It is to beunderstood, however, that the intention is not to limit the invention tothe particular embodiments described. On the contrary, the invention isintended to cover all modifications, equivalents, and alternativesfalling within the scope of the invention as defined by the appendedclaims.

DETAILED DESCRIPTION OF VARIOUS EMBODIMENTS

[0026] In the following description of the illustrated embodiments,references are made to the accompanying drawings which form a parthereof, and in which is shown by way of illustration, variousembodiments in which the invention may be practiced. It is to beunderstood that other embodiments may be utilized, and structural andfunctional changes may be made without departing from the scope of thepresent invention.

[0027] A system and method in accordance with the principles of thepresent invention generally provide for in-situ monitoring of sliderperformance in a disk drive system. In a preferred embodiment, a systemand method of the present invention provides for the detection andestimation of the resonance frequency of a slider airbearing. Changes inairbearing resonance frequency for a given slider at a particular testlocation outside of an expected range of variation is typicallyindicative of anomalous slider performance, such as a slider flying at aflyheight lower than an intended average flyheight. Unexpected changesin the airbearing resonance frequency of a given slider may be due to,for example, slider contamination, slider damage, or changes inatmospheric pressure.

[0028] A slider airbearing resonance frequency detection and estimationmethodology in accordance with the principles of the present inventionmay be implemented using existing components of a data storage system,typically requiring little or no additional hardware. As such, thefrequency detection and estimation methodology of the present inventionmay be implemented in existing and new data storage system designs, withsimple modifications typically being made only to the head positioningprocessor software. Further, software embodying the frequency detectionand estimation methodology of the present invention may be downloadedfrom a signal-bearing medium into in-service data storage systems.

[0029] Referring to the drawings, and more particularly to FIGS. 1 and2, there is illustrated a data storage system 20 within which the sliderairbearing resonance frequency detection and estimation methodology ofthe present invention may be implemented. The disk drive system 20, asis best shown in FIG. 2, typically includes one or more rigid datastorage disks 24 which are stacked coaxially in a tandem spacedrelationship, and rotate about a spindle motor 26 at a relatively highrate of rotation.

[0030] As is depicted in FIG. 1, each disk 24 is typically formatted toinclude a plurality of spaced concentric tracks 50. One or more of thedisks 24 may alternatively be formatted to include a spiraled trackconfiguration, or a combination of concentric and spiraled trackconfigurations. Digital information is typically stored in the form ofmagnetic transitions along tracks 50. Tracks 50 are generally dividedinto a number of sectors 52, with each sector 52 comprising a number ofinformation fields, including fields for storing data, and sectoridentification and synchronization information, for example.

[0031] Writing data to a magnetic data storage disk 24 generallyinvolves passing a current through the write element of the transducerassembly 27 to produce magnetic lines of flux which magnetize a specificlocation of the disk surface 24. Reading data from a specified disklocation is typically accomplished by a read element of the transducerassembly 27 sensing the magnetic field or flux lines emanating from themagnetized locations of the disk surface 24. As the read element passesover the rotating disk surface 24, the interaction between the readelement and the magnetized locations on the disk surface 24 results inthe production of electrical signals, commonly referred to as readbacksignals, in the read element.

[0032] An actuator 30 typically includes a number of interleavedactuator arms 28 with each arm having one or more transducer 27 andslider assemblies 35 mounted to a load beam 25 for transferringinformation to and from the data storage disks 24. The slider 35 istypically designed as an aerodynamic lifting body that lifts thetransducer 27 off the surface of the disk 24 as the rate of spindlemotor rotation increases and causes the transducer 27 to hover above thedisk 24 on an airbearing produced by high speed rotation of the disk 24.The distance between the slider 35 and the disk surface 24, which istypically on the order of 40-100 nanometers (nm), is commonly referredto as head-to-disk clearance or spacing.

[0033] The actuator 30 is typically mounted to a stationary actuatorshaft 32 and rotates on the shaft 32 to move the actuator arms 28 intoand out of the stack of data storage disks 24. A coil assembly 36,mounted to a coil frame 34 of the actuator 30, generally rotates withina gap 44 defined between the upper and lower magnet assemblies 40 and 42of a permanent magnet structure 38 causing the actuator arms 28, inturn, to sweep over the surface of the data storage disks 24. Thespindle motor 26 typically comprises a poly-phase AC motor or,alternatively, a DC motor energized by a power supply 46 and adapted forrotating the data storage disks 24.

[0034] The coil assembly 36 and the upper and lower magnet assemblies 40and 42 of the permanent magnet structure 38 operate in cooperation as anactuator voice coil motor 39 responsive to control signals produced by aservo processor 56. The servo processor 56 controls the direction andmagnitude of control current supplied to the voice coil motor 39. Theactuator voice coil motor 39 produces a torquing force on the actuatorcoil frame 34 when control currents of varying direction and magnitudeflow in the coil assembly 36 in the presence of a magnetic fieldproduced by the permanent magnet structure 38. The torquing forcesimparted on the actuator coil frame 34 cause corresponding rotationalmovement of the actuator arms 28 in directions dependent on the polarityof the control currents flowing in the coil assembly 36.

[0035] The data storage system 20 shown in FIG. 1 preferably employs aclosed-loop servo control system for positioning the read/writetransducers 27 to specified storage locations on the data storage disk24. During normal data storage system operation, a servo transducer,generally mounted proximate the read/write transducers, or,alternatively, incorporated as the read element of the transducerassembly 27, is typically employed to read information for the purposeof following a specified track (i.e., track following) and locating(i.e., seeking) specified track and data sector locations on the disksurface 24.

[0036] In accordance with one servo technique, embedded servo patterninformation is written to the disk 24 along segments extending in adirection generally outward from the center of the disk 24. The embeddedservo patterns are thus formed between the data storing sectors of eachtrack 50. It is noted that a servo sector typically contains a patternof data, often termed a servo burst pattern, used to maintain optimumalignment of the read/write transducers 27 over the centerline of atrack 50 when transferring data to and from specified data sectors onthe track 50. The servo information may also include sector and trackidentification codes which are used to identify the location of thetransducer assembly 27.

[0037] The servo processor 56, which cooperates with read channelelectronics 57, regulates the actuator voice coil motor 39 to move theactuator arms 28 and transducers 27 to prescribed track 50 and sector 52locations when reading and writing data to and from the disks 24. Theservo processor 56 is loosely coupled to a disk drive controller 58. Thedisk drive controller 58 typically includes control circuitry andsoftware that coordinate the transfer of data to and from the datastorage disks 24. Although the servo processor 56 and disk drivecontroller 58 are depicted as two separate devices in FIG. 1, it isunderstood that the functionality of the servo processor 56 and diskdrive controller 58 may be embodied in a single multi-purpose processor,which typically results in a reduced component cost.

[0038] Referring now to FIG. 3, there is illustrated a sectional sideview of an airbearing slider 80 which includes a lower surface 90 and atransducer element 82 mounted toward the trailing edge 88 of theairbearing surface 90. The surface 104 of data storage disk 118 is shownmoving at a velocity, V_(s), relative to the radially stationaryairbearing slider 80. A defect 105 is shown protruding upwardly from thesurface 104 of the data storage disk 118. The defect 105 is generallyrepresentative of any disk surface defect or obstruction, but will bedescribed hereinafter as an asperity 105.

[0039] It is known that asperities 105 typically arise from the surface104 of a disk 118 in a highly randomized and unpredictable manner. Amagneto-resistive transducer element 82, for example, is particularlysensitive to contact with an asperity 105 or other obstruction due inpart to its inherent sensitivity to temperature variations. Intermittentcontact between an MR transducer element 82 and asperity 105 or otherobstruction results in a temperature increase in the MR transducerelement 82, and often renders the data written at the effective disksurface location unreadable or unrecoverable.

[0040] Head-to-disk disk contact events disrupt nominal operation ofread/write transducers fabricated using other technologies. For example,a thin-film transducer element 82 is generally insensitive totemperature variations associated with asperity collisions. Manythin-film transducer elements 82 are configured to include write polesbiased with a voltage potential and are mounted near the lowerairbearing pad 93 and exposed to the disk surface 104. Intermittentcontact between a thin-film transducer element 82 and an asperity 105can result in arcing between the write poles and the disk surface 104.Such undesirable arcing frequently results in an inability to recoverdata previously written to the affected area of the disk surface 104.

[0041] Other airbearing slider configurations that incorporate opticalfiber elements at a transducer element mounting location can also suffervarying degrees of performance degradation due to abrasions to theoptical fiber probe element resulting from contact with an asperity 105.

[0042] It can be appreciated, therefore, that detecting sliders whichare flying lower than expected, and thus contacting disk surfacefeatures and defects at a greater frequency, is necessary to ensurereliable and continuous operation of a disk drive system. In addition tothe possibility of permanently losing data, repeated contact between aread/write head and disk surface asperity or defect can result inpermanent damage to the airbearing slider, which may render the head andpotentially the entire disk drive system unusable.

[0043] Still referring to FIG. 3, the data storage disk 118 typicallyrotates at a prescribed angular velocity, Ω_(D), typically on the orderof 5,000 to 8,000 RPM or higher, with the airbearing slider 80 remainingcomparatively fixed with respect to the rapidly rotating disk surface104. A typical head-to-disk contact event involves a collision betweenthe lower airbearing pad 93 of the airbearing slider 80 and an asperity105. In response to contact between an asperity 105 and the lowerairbearing pad 93, the slider 80 is displaced vertically with respect tothe surface 104 of the disk 118.

[0044] After the asperity 105 passes by the slider 80, the verticallydisplaced slider 80 follows a complex oscillatory trajectory 107 as itsettles back to its nominal flyheight, H_(N), over the disk surface 104.The contact between the asperity 105 and airbearing slider 80, andsubsequent oscillatory settling of the slider 80 results in a shorttransient sinusoidal modulation in the readback signal envelope, whichis indicative of slider airbearing resonance.

[0045] A slider airbearing resonance frequency detection and estimationapproach in accordance with an embodiment of the present inventioninvolves detecting the airbearing resonance frequency in the wake of aslider hitting a defect protruding from the surface of a data storagemedium. The airbearing frequency detection and estimation methodologyaccording to this embodiment exploits the phenomenon of spectral leakageas a means for estimating the frequency of a short transient sinusoidalsignal that results from slider contact with a protruding surfacedefect. Large changes in the airbearing resonance frequency associatedwith a given slider flying at a given track or cylinder location of adata storage disk may be used to detect and identify a poor performingslider in-situ a data storage system.

[0046] It is well understood that the presence of sinusoidal modulationin the readback signal envelope at slider airbearing resonancefrequencies is closely associated with contact between a surface of theslider and a feature protruding from a surface of a data storage medium.Experimental observations indicate that the airbearing resonancefrequency, f_(air), is an inverse nonlinear function of the flyheight,δ, and the airbearing pad-area, A. This nonlinear relationship, g(a, b),may be expressed as:

f _(air) =g(1/δ, 1/A)  [1]

[0047] By way of example, a slider having relatively large airbearingpads and operating at relatively high flyheights may have an airbearingresonance frequency of around 80 kilohertz (kHz). Airbearing slidershaving relatively small airbearing pads and flying at relatively lowerflyheights may have airbearing resonance frequencies on the order ofapproximately twice that of larger sliders. Such smaller and lowerflying sliders may have airbearing resonance frequencies of around 160kHz.

[0048] In general, a slider that is flying relatively close to the disksurface will have a higher airbearing frequency, while a slider flyingfurther from the disk surface will have a lower airbearing frequencyrelative to the close-flying slider. Further, slider airbearing padshaving relatively small surface areas are associated with higherairbearing resonance frequencies, while larger slider airbearing padareas are associated with lower airbearing resonance frequencies.

[0049] An important aspect of the present invention involves using theeffect of spectral leakage as a means of estimating the frequency of ashort transient sinusoidal signal, such as the frequency of sinusoidalmodulation induced in the readback signal envelope resulting from ahead-to-disk contact event.

[0050] It is understood that if a sinusoidal signal, x(n), does not gothrough an exact number of periods within a sampling window, then theDiscrete Fourier Transform (DFT), X(k), of the sequence has nonzerovalues for almost all values of the frequency index, k. This phenomenonis referred to as spectral leakage.

[0051] The following equation may be used to relate theanalog-to-digital sampling rate, f_(s), the number of samples, N, andthe DFT frequency index, k, to the DFT frequency, f(k):

f(k)=k·f _(s) /N=k·ω/2πN  [2]

[0052] where, f_(s) represents the sampling rate in samples per secondor Hertz, N represents the number of samples, ω represents the digitalfrequency in radians, and k represents the DFT frequency index given byk=0, 1, 2, . . . , N/2.

[0053] The phenomenon of spectral leakage is best understood withreference to FIGS. 4-9. FIG. 4 is a graphical illustration of asinusoidal signal, S₁, of frequency, f₀, sampled at sampling rate,f_(s). In this example, sampling rate, f_(s), is eight times thesinusoidal signal frequency, f₀. The graph of FIG. 4 defines a samplingwindow that spans an exact number of periods of the sinusoidal signal,S₁. In this particular example, exactly five periods of sinusoidalsignal S₁ are shown. Since the sampling window of sinusoidal signal S₁spans an exact number of signal periods, the Discrete Fourier Transformof signal S₁ exhibits only two non-zero components or spikes, as isshown in FIG. 5.

[0054]FIG. 5 depicts the magnitude of the Discrete Fourier Transform ofthe sinusoidal signal, S₁, shown in FIG. 4. FIG. 5, as well as FIGS. 7and 9, illustrates a spike at DFT components X(5) and X(35),respectively. It is noted that FIGS. 5, 7, and 9 depict the DiscreteFourier Transform of corresponding sinusoidal signals folded about theNyquist frequency value, but the information of interest is thatcorresponding to frequencies between 0 Hz and the Nyquist frequency. TheNyquist frequency is understood to represent a frequency of one-half thesampling frequency.

[0055] The data depicted in FIG. 5 illustrates a case in which nospectral leakage is present. In this case, a spike occurs at DFTcomponent X(5). This component is the most pronounced of the DFTcomponents and, therefore, contains the most power (e.g., P₀=800 in thisillustrative example). It is noted that FIG. 5 illustrates the case inwhich eight samples are obtained per period of signal S₁, and the signalsampling window spans five complete periods of the sinusoid, therebyproviding for a total of 40 DFT samples. As such, DFT component X(5) isused.

[0056] The sinusoidal signal, S₂, shown in FIG. 6, represents a signalhaving a frequency, f₁, which is different from the frequency, f₀, ofsignal S₁ shown in FIG. 4. Signal S₂, shown in FIG. 6, is sampled at thesame sampling rate, f_(s), as that of FIG. 4. In this example,sinusoidal signal, S₂, has a frequency, f₁, of 1.0333(f₀).

[0057]FIG. 7 illustrates the magnitude of the corresponding DiscreteFourier Transform of signal S₂ shown in FIG. 6. In particular, nonzerospectral components (i.e., spectral leakage) are clearly visible in FIG.7 in proximity to the spike occurring at component X(5). Spectralcomponents X(4) and X(6) represent two such nonzero spectral componentsindicative of spectral leakage associated with sinusoidal signal S₂shown in FIG. 6.

[0058] Signal S₃, shown in FIG. 8, represents a sinusoidal signal havinga frequency, f₂, equal to 0.9666(f₀). FIG. 9 depicts the magnitude ofthe corresponding Discrete Fourier Transform of signal S₃ shown in FIG.8. FIG. 9 illustrates nonzero spectral components resulting fromspectral leakage, which is clearly visible in proximity to the spikeoccurring at component X(5). As discussed previously, the spikeoccurring at component X(5) contains the most power relative to otherDFT components.

[0059] Referring to FIG. 5, it can be seen that the magnitude ofspectral component X(5) associated with sinusoidal signal S₁ shown inFIG. 4 is at a maximum (i.e., 20) when the spectral leakage is at arelative minimum. It can be further seen in FIGS. 7 and 9 that themagnitude of spectral component X(5) associated with sinusoidal signalsS₂ and S₃ respectively shown in FIGS. 6 and 8 is reduced from a maximumof 20 to approximately 19 due to the presence of neighboring nonzerospectral components. It will be appreciated that it may be difficult toidentify the maximum magnitude of the spike X(5). For example, it may bedifficult to detect changes in the magnitude of component X(5) from 20,as is shown in FIG. 5, to approximately 19, as is shown in FIG. 7, dueto the relatively small gradient in the vicinity of the maximum.

[0060] In accordance with an embodiment of the present invention, oneapproach to detecting the peak magnitude of the DFT component X(5)involves sweeping the sampling rate, f_(s), over a narrow frequencyrange and computing the value of component X(5). For purposes ofillustration, it is assumed that f_(s)=S·f_(a), where f_(a) represents afrequency that falls within the statistical range of airbearingfrequencies for a particular disk drive system design, and the parameterS represents the number of samples per average airbearing cycle. At thecoincidence of f_(a)≈f_(air), the spectral leakage will be at a relativeminimum, at which point the airbearing resonance frequency, f_(air), maybe estimated, such as by use of the methodology described in detailhereinbelow.

[0061] In order to more accurately detect the peak magnitude of the DFTcomponent X(5) and, therefore, to more accurately estimate theairbearing resonance frequency, f_(air), it has been found productive tosearch for minimal spectral leakage as the sampling rate, f_(s), isadjusted over a narrow frequency range.

[0062] As is best illustrated in FIGS. 10 and 11, the DFT componentslocated adjacent the main DFT component have sharply defined minimums.FIGS. 10 and 11 respectively illustrate the magnitudes of two DFTcomponents adjacent the main DFT component for five complete periods ofthe sinusoidal signal as a function of the ratio of sampling rate,f_(s), to constant sinusoidal frequency, f₀. FIG. 10 illustrates themagnitude of DFT component X(4), while FIG. 11 illustrates the magnitudeof DFT component X(6) in this illustrative example.

[0063]FIGS. 10 and 11 show the clear definition of the minimum for eachDFT component X(4) and X(6) occurring at R=f_(s)/f₀=8. The ratio, R, isa constant which, in this illustrative example, is given as 8, sincethere are 8 samples per period of the sinusoidal signal.

[0064] A preferred approach to searching for minimal spectral leakageinvolves comparing the magnitude of the DFT main component (e.g.,component X(5)) with the magnitude of an adjacent DFT component (e.g.,component X(4) or X(6)). In particular, it has been found useful tocompute a DFT component ratio of the magnitude of one of the adjacentDFT components (e.g., component X(4) or X(6)) to the magnitude of themain DFT component (e.g., component X(5)). DFT component ratio valuesare computed at each of a number of sampling frequencies, and theminimum of these DFT component ratio values is used to estimate theairbearing resonance frequency.

[0065] In this example, DFT component X(5) is close to its maximum valueat minimal spectral leakage, while the magnitudes of adjacent DFTcomponents X(4) and X(6) are at a minimum, respectively. For P completeairbearing resonance frequency periods, it is useful to use the DFTcomponent ratio of X(P−1)/X(P) or X(P+1)/X(P) when searching for minimalspectral leakage. Experimental results derived from use of a speciallymade bump disk drive system show that the DFT component ratioX(P+1)/X(P) yields better results when estimating the airbearingfrequency, where P represents the number of complete airbearingresonance frequency periods. It will be appreciated, however, that otherDFT component ratios may be employed, such as X(P+2)/X(P); X(P+3)/X(P);etc. It will be further appreciated that DFT component power ratios mayalso be employed, such as a DFT component power ratio of [X(P+2)/X(P)]².

[0066] Turning now to FIG. 13, there is illustrated an embodiment of anin-situ system for estimating a slider airbearing frequency, f_(air),for a disk drive system. In accordance with this embodiment, the system200 includes a disk drive assembly 206 including appropriate controlsfor controlling rotation of a magnetically recorded disk 202. Atransducer 204, flying above the surface of data storage disk 202,senses signals in the form of magnetic transitions emanating from thedisk surface 202.

[0067] The sensed readback signal, w(t), is amplified, lowpass filtered,and sampled at a sampling rate, f_(s), in block 208. The filtered andsampled readback signal, w(m), such as the filtered signal, w(m), shownin FIG. 12, is provided at the output of block 208. The amplitude of thesampled readback signal, w(m), is monitored by a defect detector andwindow selector unit 210.

[0068] A defect protruding from the surface of a data storage disk 202will yield a large detectable signal peak, such as a positive ornegative signal peak. An example of such a negative signal peakoccurring in a sampled signal, w(m), provided at the output of block 208can be seen in FIG. 12. If a negative threshold is exceeded in thesampled signal, w(m), the defect detector and window selector 210 isactivated. A delayed windowed section immediately following the negativepeak is captured and stored in a RAM storage unit 112, which isactivated via line 211. The delay before the data capture is roughly 4microseconds. The delayed windowed section immediately following thenegative peak contains the best detectable portion of the airbearingresonance event. Similar delayed windows may be based on the positivesignal peak.

[0069]FIG. 12 illustrates the large detectable negative signal peakresulting from a protruding defect developed on the surface of disk 202.FIG. 12 further illustrates a sampling window section immediatelyfollowing the negative peak. The width of the window section should bewide enough to capture P complete airbearing periods. If the typicalsize of the RAM storage unit 212 is 128 bytes, for example, then eightcomplete periods at 16 samples per period may be stored in RAM storageunit 212.

[0070] The discrete signal segment, w(m), is then upsampled by a factorof Q in block 222 and later downsampled in block 224. The upsampling isimplemented by inserting Q−1 zeros between each sample of signal w(m).In block 222, the upsampled discrete signal sequence is interpolated andamplified (scale Q) by a lowpass filter having a cutoff frequency atapproximately 1.5 times the highest anticipated airbearing frequency,f_(a). It is noted that an interpolation filter may be operated in “Javamode” for purposes of minimizing storage. It is further noted that onlythe downsampled values of y(r) are used and stored as x(n) values,whereas other values of y(r) are ignored.

[0071] The upsampling factor Q is approximately 100-200, so that theinterpolation filter output, y(r), has 100-200 times the number ofsamples associated with discrete signal segment w(m). However, in thedownsampling unit 224, only a fraction, 1/N_(i), of these samples needto be stored. The RAM storage requirement with respect to block 224 aretherefore less than that for block 212.

[0072] The downsampling is accomplished in an iterative mode in block224 within process loop 220. The range of downsampling factors, N_(i),is determined from the known or modeled statistical range of airbearingresonance frequencies, f_(min)≦f_(a)≦f_(max). Computing the range of thedownsampling factor, N_(i), may be accomplished using Equation 3 below:$\begin{matrix}{\frac{{Qf}_{s}}{{Sf}_{\max}} \leq N_{i} \leq \frac{{Qf}_{s}}{{Sf}_{\min}}} & \lbrack 3\rbrack\end{matrix}$

[0073] For purposes of illustration, if it is assumed that f_(s)=1.25MHz, f_(min)=120 kHz, f_(max)=180 kHz, Q=200, and S=16, the range of theincremental downsampling factors, N_(i), to be stored in memory, such asin the form of a look-up table, would be given as 87≦N_(i)≦130. In thisillustrative example, the number of downsampling values, M, for testingpurposes is M=130−87+1=44.

[0074] Incrementing N_(i) successively by one in the rangeN_(min)≦N_(i)≦N_(max) may be used in the iterative process loop 220. Aconquer-and-divide approach may be desirable due to the lack ofuniformity in the DFT component ratio, R_(i), as the downsampling factorN_(i) is changed.

[0075] The downsampled values x(n) for each value of N_(i) aretemporarily stored in unit 224 such that the magnitudes of α-pointDiscrete Fourier Transforms, X_(α)(P), X_(α)(P+1), may be computed usingGoertzel's individual DFT component method in block 226, from which theDFT component ratio R_(i) is computed. The DFT component ratio, R_(i),may be expressed as: $\begin{matrix}{R_{i} = \frac{\left| {X_{\alpha}\left( {P + 1} \right)} \right|}{\left| {X_{\alpha}(P)} \right|}} & \lbrack 4\rbrack\end{matrix}$

[0076] where, α=P·S, P represents the number of complete airbearingresonance frequency periods, and S represents the number of samples perperiod. The DFT component ratio, R_(i), with its corresponding N_(i)value is stored in block 228, and the downsampling factor N_(i) isincremented by one in block 230. This process is then repeated beginningat upsampling and interpolation block 222 until all M values of N_(i)have been exhausted.

[0077] The final steps to estimating the airbearing resonance frequency,f_(air), are performed in blocks 232 and 234. Using the DFT componentratios, R_(i), and corresponding N_(i) values stored in block 228, thevalue of {overscore (N)}_(i) for which the DFT component ratio, R_(i),is at a minimum is computed in block 232. The computation and monitorunit 234 then estimates the airbearing resonance frequency, f_(air),using the following equation:

{circumflex over (f)} _(air)=(Qf _(s))/(S{overscore (N)} _(i))  [5]

[0078] Any significant changes in the estimated airbearing resonancefrequency, f_(air), at a given test cylinder suggests appreciableunintended changes in slider flyheight. Such changes may be reported toa predictive failure analysis (PFA) system of the disk drive system.

[0079] When several periods, P, are included in the DFT component ratioestimate, the average value of the slider airbearing resonance frequencymay be obtained. It is noted that due to the available signal-to-noiseratio, the damping of the sinusoidal airbearing resonance signal alsolimits the number of periods, P, that can be used.

[0080] The above-described slider airbearing resonance frequencyestimation methodology was simulated using commercially available DSPsoftware (e.g., MATLAB). Test results on simulated and actual data showvery good results. By way of example, for the test bump disk drivesystem sequence shown in FIG. 12, where f_(s)=1.25 MHz, Q=200, P=3, andS=16, the minimum DFT component ratio (i.e., minimum spectral leakage)was computed as R_(min)=0.1604, and the estimated airbearing resonancefrequency, f_(air), was computed as f_(air)=149 kHz. Another verysimilar test bump disk drive system sequence produced an estimatedairbearing resonance frequency, f_(air), of f_(air)=163 kHz.

[0081] It is understood in the art that the Discrete Fourier Transformmay be employed for spectral analysis of a finite-length signal composedof sinusoidal harmonics, or components, as long as the frequency,amplitude, and phase of each sinusoidal harmonic component issubstantially time-invariant and independent of the sequence length. Ingeneral, the airbearing resonance frequency, f_(air), is nonlinear, andis not entirely stationary over a given observation period. Themodulation produced in the readback signal by the airbearing resonancehas a frequency, amplitude, and phase that are slightly time-varying ornon-stationary over the sequence length. However, an assumption oftime-invariance of the lower harmonic components in the DFT is generallyvalid.

[0082] To further address the non-stationary character of the readbacksignal, an alternative to a DFT approach involves segmenting thesequence into a set of sub-sequences of shorter length, with eachsubsequence centered at uniform intervals of time and having its DFTcomputed separately. This method involves the use of a time-dependentDFT or Short-Time DFT (STFT). The Short-Time Fourier Transform uses aHamming, Hanning, or other symmetric windows in the frequency transformto extract a finite-length portion of the sequence, such that thespectral characteristics of the extracted section are approximatelystationary over the duration of the window.

[0083] Both DFT and STFT frequency transformation approaches may beused, however, due to the typically very short duration of theairbearing resonance. For an airbearing resonance frequency of 80 kHz,by way of example, there are about 80 samples during five airbearingresonance periods at 1.25 MHz. These 80 samples may be multiplied by asymmetric window before the DFT is computed.

[0084] A system and methodology in accordance with the principles of thepresent invention provide for estimating the frequency of a sliderairbearing by searching for the minimal spectral leakage associated withairbearing resonance. The methodology is used in one embodiment toindirectly estimate changes in slider/transducer flyheight for eachindividual read/write head. A large variation in airbearing resonance ata given test cylinder is indicative of a major change inslider/transducer flyheight. For example, a large increase, such as onthe order of 10% or higher, in the airbearing resonance frequency at agiven test cylinder suggests that the slider/transducer is flying lowerthan during previous operation. This change in flyheight may be used toassess the general operating health of the head-to-disk interface (HDI).For example, a significant reduction in slider/transducer flyheight mayindicate the presence of slider contamination, slider damage, or achange in atmospheric pressure.

[0085] The system and method of the present invention may also be usedto operate with the position error signal (PES) in order to identify anddetermine suspension resonance problems. The methodology of the presentinvention requires a minimal amount of hardware resources, and may beused as a predictive failure analysis tool during in-situ disk drivesystem operation. Both the magnetic and thermal portions of the readbacksignal may be used. It is understood that the system and method of thepresent invention may be used to detect and estimate the frequency ofany signal containing a sinusoidal component whose frequency is known tofall within a given frequency range, and is not limited to use inconnection only with readback signals and position error signalsassociated with disk drive system operation.

[0086] The airbearing frequency estimation methodology of the presentinvention, as previously discussed, requires little or no additionalhardware to implement in existing and future disk drive systems. Theservo processor software may be modified to effect the process stepsdescribed with respect to the embodiment depicted in FIG. 13. Anairbearing frequency estimation methodology according to the presentinvention may thus be effected, for example, by the controllerimplementing a sequence of machine-readable instructions. Theseinstructions may reside in various types of signal-bearing media.

[0087] In this respect, another embodiment of the present inventionconcerns a programmed product which includes a signal-bearing mediumembodying a program of machine-readable instructions, executable by adigital processor to perform method steps to effect an airbearingfrequency estimation procedure. The signal-bearing media may include,for example, random access memory (RAM) provided within, or otherwisecoupled to, the servo processor or arm electronics module.

[0088] Alternatively, the instructions may be contained in othersignal-bearing media, such as one or more magnetic data storagediskettes, direct access data storage disks (e.g., a conventional harddrive or a RAID array), magnetic tape, alterable or non-alterableelectronic read-only memory (e.g., EEPROM, ROM), flash memory, opticalstorage devices (e.g., CDROM or WORM), signal-bearing media includingtransmission media such as digital, analog, and communication links andwireless, and propagated signal media. In an illustrative embodiment,the machine-readable instructions may constitute lines of compiled “C”language code or “C++” object-oriented code.

[0089] The foregoing description of the various embodiments of theinvention has been presented for the purposes of illustration anddescription. It is not intended to be exhaustive or to limit theinvention to the precise form disclosed. For example, a variety ofdifferent frequency transform techniques may be employed, includingDiscrete Fourier Transform, Fast Fourier Transform (FFT), and Short-TimeDFT (STFT) techniques. One of several frequency transform approaches maybe implemented depending on whether the detected airbearing signal isstationary or non-stationary over time. Many modifications andvariations are possible in light of the above teaching. It is intendedthat the scope of the invention be limited not by this detaileddescription, but rather by the claims appended hereto.

What is claimed is:
 1. A method of estimating a value of a resonance frequency of an airbearing associated with a slider flying in proximity to a data storage medium, the method comprising: obtaining a readback signal from the data storage medium; and estimating the value of the airbearing resonance frequency using the readback signal.
 2. The method of claim 1, wherein estimating the value of the airbearing resonance frequency comprises estimating the value of the airbearing resonance frequency using the readback signal obtained from the data storage medium over a plurality of complete airbearing periods.
 3. The method of claim 1, wherein estimating the value of the airbearing resonance frequency comprises using spectral leakage associated with a frequency transform of the readback signal to estimate the value of the airbearing resonance frequency.
 4. The method of claim 1, wherein estimating the value of the airbearing resonance frequency comprises using a frequency transform of the readback signal, the frequency transform obtained using a Discrete Fourier Transform (DFT), a Fast Fourier Transform (FFT), or a Short-Time Discrete Fourier Transform (STFT) of the readback signal.
 5. The method of claim 1, wherein estimating the value of the airbearing resonance frequency comprises using a Discrete Fourier Transform (DFT) of the readback signal in accordance with Goertzel's algorithm.
 6. The method of claim 1, wherein estimating the value of the airbearing resonance frequency comprises: producing, using the readback signal obtained from the data storage medium over a plurality of complete airbearing periods, a discrete signal segment comprising a plurality of frequency transform components; and estimating the value of the airbearing resonance frequency using spectral leakage of the discrete signal segment.
 7. The method of claim 6, wherein the discrete signal segment is produced in response to detecting contact between the slider and a feature protruding from a surface of the data storage medium.
 8. The method of claim 1, wherein estimating the value of the airbearing resonance frequency comprises: producing, using the readback signal obtained from the data storage medium over a plurality of complete airbearing periods, a discrete signal segment comprising a plurality of frequency transform components; computing a ratio of a magnitude of a first component to a magnitude of a second component at each of a plurality of sampling rates, the second component related to the resonance frequency of the airbearing; and estimating the value of the airbearing resonance frequency using a minimum of the ratios.
 9. The method of claim 8, wherein the first component is a component adjacent the second component.
 10. The method of claim 8, wherein the first component is a non-adjacent component relative to the second component.
 11. The method of claim 8, wherein the sampling rates are defined by a number of samples per average airbearing period multiplied by a frequency defined within a range of expected airbearing frequencies.
 12. The method of claim 8, wherein the ratios are power ratios.
 13. The method of claim 1, wherein the readback signal comprises a magnetic signal component, a thermal signal component, or magnetic and thermal signal components.
 14. An apparatus for estimating a value of a resonance frequency of an airbearing, comprising: a data storage medium; a transducer provided on a slider and producing a readback signal obtained from the data storage medium; and a processor that receives the readback signal from the transducer and estimates the value of the airbearing resonance frequency using the readback signal.
 15. The apparatus of claim 14, wherein the processor estimates the value of the airbearing resonance frequency using spectral leakage associated with a frequency transform of the readback signal.
 16. The apparatus of claim 14, wherein the processor estimates the value of the airbearing resonance frequency using one of a Discrete Fourier Transform (DFT), a Fast Fourier Transform (FFT), or a Short-Time Discrete Fourier Transform (STFT) of the readback signal.
 17. The apparatus of claim 14, wherein the processor estimates the value of the airbearing resonance frequency using a Discrete Fourier Transform (DFT) of the readback signal in accordance with Goertzel's algorithm.
 18. The apparatus of claim 14, wherein the processor estimates the value of the airbearing resonance frequency by: producing, using the readback signal obtained from the data storage medium over a plurality of complete airbearing periods, a discrete signal segment comprising a plurality of frequency transform components; computing a ratio of a magnitude of a first component to a magnitude of a second component at each of a plurality of sampling rates, the second component related to the resonance frequency of the airbearing; and estimating the value of the airbearing resonance frequency using a minimum of the ratios.
 19. The apparatus of claim 18, wherein the first component is a component adjacent to or non-adjacent to the second component.
 20. The apparatus of claim 18, wherein the ratios are power ratios.
 21. The apparatus of claim 18, wherein the readback signal comprises a magnetic signal component, a thermal signal component, or magnetic and thermal signal components.
 22. The apparatus of claim 18, wherein the processor varies a sampling rate of the discrete signal sample.
 23. The apparatus of claim 14, further comprising a detection circuit, the detection circuit detecting a change in an amplitude of the readback signal indicative of contact between the slider and a feature protruding from a surface of the data storage medium.
 24. The apparatus of claim 14, wherein the processor estimates the resonance frequency of the airbearing using instructions contained in a signal-bearing media.
 25. A data storing system, comprising: a data storage disk; a transducer provided on a slider; an actuator for providing relative movement between the slider and the disk; and a processor that receives a readback signal from the transducer and estimates a value of the airbearing resonance frequency using the readback signal.
 26. The system of claim 25, wherein the processor estimates the value of the airbearing resonance frequency using spectral leakage associated with a frequency transform of the readback signal.
 27. The system of claim 25, wherein the processor estimates the value of the airbearing resonance frequency using one of a Discrete Fourier Transform (DFT), a Fast Fourier Transform (FFT), or a Short-Time Discrete Fourier Transform (STFT) of the readback signal.
 28. The system of claim 25, wherein the processor estimates the value of the airbearing resonance frequency using a Discrete Fourier Transform (DFT) of the readback signal in accordance with Goertzel's algorithm.
 29. The system of claim 25, wherein the processor estimates the value of the airbearing resonance frequency by: producing, using the readback signal obtained from the data storage disk over a plurality of complete airbearing periods, a discrete signal segment comprising a plurality of frequency transform components; computing a ratio of a magnitude of a first component to a magnitude of a second component at each of a plurality of sampling rates, the second component related to the resonance frequency of the airbearing; and estimating the value of the airbearing resonance frequency using a minimum of the ratios.
 30. The system of claim 29, wherein the ratios are power ratios.
 31. The system of claim 25, wherein the readback signal comprises a magnetic signal component, a thermal signal component, or magnetic and thermal signal components.
 32. The system of claim 25, wherein the processor estimates the resonance frequency of the airbearing using instructions contained in a signal-bearing media. 